a) Field of the Invention
The present invention relates to an image pickup apparatus, and more specifically an image pickup apparatus which uses a composite filter.
b) Description of the Prior Art
It is general to use an infrared cut filter in combination with a single plate color CCD in order to cut detrimental near infrared rays.
Known as infrared cut filters are an absorption type filter which absorbs rays having specific wavelengths dependently on concentrations of pigments and a reflection type filter using an interference film which cancels only rays having specific wavelengths by utilizing a phase difference.
Out of these infrared cut filters, the absorption type infrared cut filter must have a certain definite thickness to obtain a specific spectral characteristic dependently on the concentrations of the pigment. Accordingly, the absorption type infrared cut filter is not always optimum for an image pickup apparatus which is restricted in a longitudinal direction, an endoscope or the like in particular. Furthermore, the absorption type infrared cut filter has a cutoff characteristic which traces a gentle curve and is not preferable from a viewpoint of a color reproducibility.
On the other hand, the reflection type filter is less restricted in the longitudinal direction and has a steep cutoff characteristic, thereby having a relatively high color reproducibility. However, the reflection type filter has a highly reflective coated surface, and may produce flare and ghost.
A composite filter is known as a filter which utilizes merits and makes up for defects of the filters. Japanese Utility Model Kokai Publication No. Hei 1-125417 (U.S. Pat. No. 5,177,605) discloses an example of composite filter shown in FIGS. 1A and 1B which is composed of a combination of an absorption type filter and a reflection type filter to enhance a color reproducibility and reduce ghost. In FIG. 1A, a reference symbol F(A) represents the absorption type filter and a reference symbol F(R) designates the reflection type filter, and in FIG. 1B, a reference symbol T[F(A)] denotes spectral transmittance of the absorption type filter, a reference symbol T[F(R)] represents spectral transmittance of the reflection type filter, a reference symbol T[f(A)F(R)] designates total spectral transmittance, a reference symbol T denotes transmittance and a reference symbol W represents a wavelength.
This filter mentioned as a conventional example lowers an intensity of ghost rays which are produced by multiple reflections between the reflection type filter and another highly reflective surface with the absorption type filter, and corrects a cutoff characteristic of the absorption type filter with a steep cutoff characteristic of the reflection type filter.
However, it is actually impossible to obtain a function which is effective to satisfy a requirement described above simply by combining different kinds of filters as in the conventional example. Furthermore, the utility model mentioned above makes no disclosure of a concrete filter which is effective to satisfy the requirement described above nor a characteristic of the filter.
Though the utility model which discloses the conventional example describes problems of color reproducibility, ghost and the like which are attributed to the filters themselves, it makes no reference to a characteristic of a CCD used in combination with the composite filter. Optimalization of an infrared filter is largely different dependently on a characteristic of a CCD used in combination therewith, that of a color mosaic filter in particular.
A single plate color filter generally prepares color difference signals with four vertical lines as shown in FIG. 2, and each line of a mosaic filter is composed by combining yellow Ye, cyanic Cy, magenta Mg and green G as listed below:                First line: Ye, Cy, Ye, Cy        Second line: Mg, G, Mg, G        Third line: Ye, Cy, Ye, Cy        Fourth line: G, Mg, G, Mg        
An n-th color difference signals (R−Y, B−Y) in an A field are prepared by Cy and Ye on the first line, and G and Mg on the second line, whereby R−Y=2R−G.
Furthermore, an (n+1)th color difference signals in the A field are prepared by Cy and Ye on the third line, and G and Mg on the fourth line, whereby B−Y=2B−G.
The reference symbol R represents a red signal, the reference symbol G designates green signal, the reference symbol B denotes a blue signal and the reference symbol Y represents a luminance signal.
When an intense light is incident on a CCD to set it in a saturated condition, signal levels of the color signals R, G and B are set at 1:1:1. However, output signals are not always set at 1:1:1 since a spectral characteristic of the CCD itself has a peak in an infrared region. That is, only a signal from the nth line in the S field including the color signal R is set at a high level. As a result, noise appears on a monitor as red light and shade.
An infrared cut filter is usually used to adjust a level of the red signal thereby preventing such noise to be produced.
In case of an instrument such as a medical endoscope which is used to observe an interior of a human body in particular, an organ to be observed is mainly reddish. Accordingly, the horizontal stripe noise is liable to be noticeable. Furthermore, such an endoscope uses a light source which emits a light containing infrared spectral components in amounts relatively larger than those in natural light and fluorescent light, thereby inevitably requiring an infrared cut filter.
Though it is possible to cancel or reduce such noise by multiplying an output signal by a certain coefficient, needless to say, it is inevitably necessary for this purpose to use an exclusive circuit and customize a CCD. A manufacturing cost of an image pickup apparatus is enhanced in this case.
In a field of medical treatments, many imaging systems for public use have recently been adopting DSPs (digital signal processors) which have high performance and are manufactured at low costs. The DSPs themselves are frequently adopted for appliances which process large amounts of information such as compressed voice and compressed images to exhibit their effects for accelerating processing speeds. In these cases, signals from the CCDs are processed dependently on specifications for the DSPs.
In case of an endoscope system which handles a plurality of endoscopes using different CCDs, it is naturally difficult to customize signal processing circuits which are specialized for individual fields or individual instruments. Accordingly, it is obliged to rely on infrared cut filters as means to reduce noise such as that described above.
The description of the characteristics of the CCDs is not provided in Japanese Utility Model Kokai Publication No. Hei 1-125417 (U.S. Pat. No. 5,177,605).
In an endoscope system which controls a plurality of endoscopes with a single CCU (camera control unit), specifications for optimum endoscopes are different dependently on fields of products of optical instruments. The inexpensive absorption type infrared cut filter is mainly used, for example, in a medical endoscope for digestive systems or external TV sets for which a relatively loose restriction is imposed on a size of an endoscope. In contrast, the reflection type infrared cut filter which is advantageous for compact configuration is used in an endoscope for bronchi for which a relatively severe restriction is imposed on the size of the endoscope. Furthermore, a CC to be used in an endoscope is selected so as to be optimum for a field of use.
Different kinds of filters and different kinds of CCDs are selected for the endoscope which are used in the endoscope system as described above. Accordingly, a color reproducibility of the endoscope system is different dependently on endoscope used in the endoscope system and it may be difficult to correct this difference with the CCU.